1. Field
The present application relates generally to substrate processing and, more particularly, to methods and apparatuses for transferring substrates through a substrate processing system.
2. Description of the Related Art
An apparatus for processing semiconductor substrates, such as silicon wafers, typically includes a processing chamber in which the substrates are processed, a substrate handling chamber through which the substrates are moved before and after processing in the processing chamber, and one or more input/output chambers that store substrates before and after the substrates are moved through the handling chamber. A substrate transfer robot is located within the handling chamber and is configured to transfer substrates to and from a plurality of stations. Such stations may be within the handling chamber, the input/output chambers, the processing chamber, or other chambers. A typical station within the processing chamber is a substrate holder, such as a susceptor, which supports a substrate during processing. A station within the input/output chamber may comprise a cassette that holds a plurality of substrates. The input/output chambers may be loading chambers or load ports that contain substrate cassettes that are accessible by the transfer robot. The input/output chambers can also be load-lock chambers in which substrates can be atmospherically isolated and purged of particulates before being moved into the handling chamber and eventually into the processing chamber. Other stations, which can be inside separate chambers or even within the substrate handling chamber, may include pre-processing stations (such as a wafer pre-cleaning station) and/or post-processing stations (such as a cooling station).
The substrate transfer robot typically includes an actuator, one or more interlinked arms, and an end effector attached to the arms. The actuator is configured to move the arms and the end effector. The end effector is adapted to pick up a substrate from a station, hold the substrate as the robot moves the end effector and the substrate to another station, and place the substrate at another station. A variety of different types of end effectors exist, including paddles and Bernoulli wands.
In so-called cluster tools, the apparatus includes a plurality of processing chambers each typically adjacent to the substrate handling chamber. The processing chambers are capable of processing substrates simultaneously, which increases the overall substrate throughput of the apparatus. The handling chamber may include more than one substrate transfer robot for improved substrate handling capability.
Substrates must often be placed with great accuracy. A typical susceptor for holding a semiconductor wafer has a wafer pocket whose size is only slightly larger than that of a wafer that the susceptor is designed to support. For example, a susceptor designed to support a 300 mm wafer might have a wafer pocket with a diameter of only 301 mm, providing a clearance of only 0.5 mm between the edge of the supported wafer and the pocket's perimeter wall. It is important that the wafer be centered in the pocket and not touch the pocket wall. If the wafer contacts the pocket wall, local temperature changes occur, resulting in temperature gradients across the wafer. This can cause non-uniformity in process results, as most semiconductor processing depends critically on temperature.
Errors in placement of the substrate relative to an end effector, known as “substrate drift,” are sometimes caused by variations in substrate position in the cassette at pickup. In other words, at pickup the end effector may attach to each substrate at a slightly different location. Substrate drift can also occur during robot movement, particularly when the robot moves fast. In addition to the aforementioned problems associated with failing to center the substrate within a susceptor pocket, substrate drift can result in damage to the substrate during placement at a substrate station, such as a wafer cassette.
One method of solving these problems associated with substrate drift involves the use of at least two photosensors in the wafer handling chamber. For example, U.S. Pat. No. 7,008,802 discloses using photosensors that sense an extent to which an edge of the wafer blocks a light beam emitted by the photosensor. By measuring the extent to which the wafer edge blocks the light beams of the photosensors, the position of the wafer is calculated and compared to a pre-registered normal position to calculate an offset displacement to be applied to the robot to compensate for the calculated positioning error. Another reference that discloses using two photosensors to calculate a position of a wafer is Japanese Patent Laid-open No. 2005-93807.